Fluorescent lamps comprise an elongated tubular envelope having a pair of electrodes sealed into each end thereof. The interior surface of the envelope is coated with a finely divided fluorescent material, and the envelope is filled with a gaseous atmosphere, such as of a rare gas and a metal vapor. In operation, the electrodes create an electrical discharge which in turn emits ultraviolet radiation that excites the fluorescent material, causing it to glow.
Such fluorescent lamps are made by suspending suitable phosphor particles and a suitable binder in a solvent, flushing the interior surface of the lamp envelope with the resultant suspension, draining off the excess suspension, and heating the interior wall of the envelope to remove the solvent and organic materials. An adherent coating of the phosphor material now covers the interior walls of the lamp envelope.
Yttrium-containing phosphors can be used alone, or along with other phosphors, as is known. For example, triphosphor-coated fluorescent lamps have the advantage of well-resolved spectral lines, which provide better lighting to the human eye than standard cool-white halophosphate lamps, which have a broad spectral emission. In these lamps, the three phosphors applied to lamps are a red phosphor, generally yttrium europium oxide (Y.sub.2 O.sub.3 :Eu), a green phosphor, usually either CAT [(Ce,Tb)MgAl.sub.11 O.sub.19 ] or lanthanum phosphate (LaPO.sub.4 :Tb,Ce), and a blue phosphor, either BAM (BaMg.sub.2 Al.sub.27 :Eu) or strontium phosphate (Sr.sub.5 (PO.sub.4).sub.3 Cl:Eu). The relative amounts of the red, green and blue phosphor are adjusted to achieve the desired lamp operating temperature (color point).
Although we have illustrated the use of yttrium europium oxide phosphors in one particular family of lamps, the phosphor of the invention can be used equally in other lamp types, such as a two-component single coat phosphor blend of the narrow band yttrium oxide:Eu phosphor with a broad band strontium yellow or strontium green phosphor. Furthermore, the narrow band red, green and blue phosphors may be applied as a second coat over a layer of the broad band white halophosphor, or as a thicker, single coat of phosphor.
Yttrium-containing phosphors and mixtures of phosphors can be applied from a non-aqueous slurry, for example based on a nitrocellulose binder, in organic solvents such as butyl acetate, which provides stable phosphor slurries. However, the use of organic solvents is expensive, creates fire hazards and is subject to strict environmental regulation as well, and it would be highly desirable to substitute aqueous slurries in place of the organic solvent slurries. However, aqueous phosphor slurries that contain yttrium-containing phosphors sometimes prove unstable; they can exhibit poor chemical stability and poor coating appearance. Particularly in the presence of polymeric binders that contain reactive chemical groups, such as hydroxyl and carboxyl groups, e.g., polyacrylic acid, the slurries form a non-pourable gel within a few days. These gels cannot be reconstituted, and thus the phosphor must be reclaimed laboriously. When these aqueous phosphor/binder compositions are used as coatings in fluorescent lamps, severe flocculation of the phosphor can also occur, resulting in objectionable appearance defects in the coated lamps.
Further, the instability and gelation time varies from batch to batch of the phosphor, creating uncertainty in the behavior and stability of each batch of phosphor, which cannot be tolerated in a modern manufacturing facility.
Thus improved yttrium-containing phosphors and mixtures thereof in aqueous slurries having improved stability and resistance to gelation and flocculation, a method of obtaining the same, and of producing fluorescent lamps using the improved slurries, would be highly desirable.